McDonough was sentenced to serve between 3.5 and 7 years on each charge -- a potential prison term of up to 21 years — but a majority of that time was suspended.

The sentence came as part of a plea bargain with the state, which will require her to testify in the pending murder trial of her former boyfriend, Seth Mazzaglia.

More than 20 of Marriott's family members and friends attended the hearing. Many delivered emotional impact statements, and some also expressed displeasure with the prospect that McDonough could be released from jail in just three years time.

Judge Marguerite Wageling accepted the plea deal, but emphasized to McDonough before handing down the sentence that she is culpable for the Marriott family's pain.

"But for your cowardly and selfish actions, (Elizabeth Marriott) would be alive, and/or this family would have the body to lay it to rest,” she said. “But for you, they would have that peace, and you will carry that around in your conscience for the rest of your life."

Marriott disappeared on Oct. 9, 2012, after attending classes at the University of New Hampshire. Prosecutors say she was strangled to death with a rope that night by McDonough's boyfriend, Seth Mazzaglia. The murder took place inside an apartment Mazzaglia shared with McDonough in Dover, police say.

McDonough initially told police Marriott never showed up at the apartment on Oct. 9, even though she was planning to visit. But that was a lie, McDonough admitted in court Thursday, concocted to throw investigators off track.

McDonough also admitted that on Nov. 7, 2012 -- nearly a month after Marriott was killed -- McDonough tried to persuade a Rochester woman who saw Marriott's body to lie to police.

McDonough entered guilty pleas on all three charges Thursday, and also waived her right for a sentence review. She agreed the state had sufficient evidence to prove her guilt beyond a reasonable doubt on each charge.

"Kathryn McDonough, someday I hope that you can look back and face this tragedy and acknowledge to yourself your part in it,” Marriott's father, Bob Marriott, said during a tearful statement to the judge. “You had the chance to do the right thing — to try to help, to do something heroic. Your failure in that moment is why Lizzi is not here to live out her life."

Mazzaglia told investigators he put a rope around Marriott's neck while he was having intercourse with her, and applied pressure to the rope. At some point, McDonough also became involved, he said, but it was Mazzaglia who was tightening the rope before Marriott's death.

Mazzaglia said Marriott suffered what he described as a seizure. Mazzaglia said he then checked her body for a pulse in multiple locations and placed a plastic grocery bag over her head.

A man and woman from Rochester also reportedly visited Mazzaglia's apartment later that night, sometime after Marriott fell unconscious. In interviews with police, they recalled seeing a woman lying on his living room floor, clothed only in a pair of blue underwear, with brown plastic grocery bags covering her head and face.

Mazzaglia and McDonough allegedly used Marriott's car to transport Marriott's body to Peirce Island in Portsmouth and discard her remains into the Piscataqua River that same evening.

Prosecutors say 29 days after the killing, McDonough contacted the Rochester woman and asked her to lie about visiting Mazzaglia's apartment.